1. Field of Invention
The present invention relates to strong glass-ceramics materials containing miserite crystals as the predominant crystalline phase.
2. Description of the Related Art
Glass-ceramics are polycrystalline materials formed by a controlled crystallization of a precursor glass article. A glass-ceramic may be prepared by exposing a glass monolith to a thermal treatment for conversion to a crystalline state. This is referred to as a "bulk or monolith forming process". Monolith glass-ceramic forming technology is founded in U.S. Pat. No. 2,920,971 (Stookey). In general, raw materials usually containing a nucleating agent are melted and simultaneously cooled to form a glass monolith of desired geometry. Thereafter the glass monolith is exposed to a crystallizing thermal treatment referred to as "ceramming". The appropriate thermal treatment typically includes a low temperature hold at a temperature somewhere above the transformation range to induce nucleation, followed by one or more higher temperature holds at temperatures somewhere above the softening point to promote crystal growth. In the monolith forming process nucleation transpires internally. The manufacture of glass-ceramics by bulk forming processes is compatible with high-speed, automated manufacturing processes employed in the formation of glass articles. Furthermore, internal nucleation can provide a wide range of potential crystalline microstructures.
U.S. Pat. Nos. 4,386,162 and 4,397,670 (Beall) discloses the preparation of glass-ceramic articles demonstrating high mechanical strength and toughness wherein crystals of canasite and/or agrellite and/or fedorite having a chain silicate structure constitute the predominant crystal phase. Those articles have a composition expressed in terms of weight percent on the oxide basis, comprising
______________________________________ SiO.sub.2 45-75 Na.sub.2 O 3-15 B.sub.2 O.sub.3 0-6 CaO 8-30 K.sub.2 O 0-20 Al.sub.2 O.sub.3 0-7 F 3.5-12 Na.sub.2 O + K.sub.2 O 5-25 ZrO.sub.2 0-12. ______________________________________
Compositions resulting in glass-ceramics exhibiting the greatest mechanical strength and toughness containing canasite as essentially the sole crystal phase have a composition expressed in terms of weight percent on the oxide basis, comprising
______________________________________ SiO.sub.2 50-65 Na.sub.2 O 3-13 B.sub.2 O.sub.3 0-3 CaO 15-24 K.sub.2 O 3-15 Al.sub.2 O.sub.3 0-3 F 5-9 Na.sub.2 O + K.sub.2 O 11-22 ZrO.sub.2 0-8. ______________________________________
Fluorite crystals (CaF.sub.2) provide the nuclei upon which the canasite crystals [Ca.sub.5 Na.sub.4 K.sub.2 (Si.sub.12 O.sub.30)F.sub.4 ] grow. Where canasite comprises essentially the sole crystal phase, there is an interlocking blade-like crystalline morphology, which structure has conferred modulus of rupture values to the crystallized articles in excess of 50,000 psi (345 MPa).
U.S. Pat. No. 4,467,039 (Beall et al.) discloses the formation of glass-ceramic articles exhibiting high strength and toughness containing potassium fluorrichterite (KNaCaMg.sub.5 Si.sub.8 O.sub.22 F.sub.2) having a chain silicate structure as the predominant crystal phase.
The precursor glasses demonstrate the capability of ceramming very rapidly upon heat treatment and have a composition expressed in terms of weight percent on the oxide basis, comprising
______________________________________ SiO.sub.2 50-70 F 3-8 Li.sub.2 O 0-3 CaO 4-15 Na.sub.2 O 2-9 Al.sub.2 O.sub.3 0-7 MgO 8-25 K.sub.2 O 2-12. ______________________________________
Whereas the crystalline articles may also contain potassium-rich canasite (K.sub.3 Na.sub.3 Ca.sub.3 Si.sub.12 O.sub.30 F.sub.4), the preferred products contain potassium fluorrichterite as essentially the sole crystal phase, and have a composition expressed in terms of weight percent on the oxide basis, comprising
______________________________________ SiO.sub.2 57-68 CaF.sub.2 7-12 K.sub.2 O 3-7 Al.sub.2 O.sub.3 0-4 Na.sub.2 O 2.5-5 MgO 14-18 CaO 0-3. ______________________________________
A glass-ceramic may also be prepared by firing glass frits in what is referred to as powder processing methods. A glass is reduced to a powder state, formed to a desired shape, fired and crystallized to a glass-ceramic state. Instead of using internal nucleation, the relict surfaces of the glass grains serve as nucleating sites for the crystal phases. The glass composition, particle size, and processing conditions are chosen such that the glass softens prior to crystallization and undergoes viscous sintering to maximum density just before the crystallization process is completed. Shape forming methods may include but are not limited to extrusion, slip-casting, tape casting, spray drying and isostatic pressing.
Examples of powder processing glass-forming technology can be found in U.S. Pat. Nos. 3,839,001 (Adams et al.), 4,464,475 (Beall et al.), and 5,001,086 (Carrier et al.), herein incorporated by reference.
The manufacture of sintered glass-ceramics is compatible with glass-ceramic compositions for which no nucleating agents exist and is particularly suitable for the production of highly complex and intricately-shaped articles.
It is somewhat uncommon for any one glass-ceramic composition to be compatible with both the monolith and powder forming processes. Surface crystallization and poor internal nucleation in a monolith leads to very weak, deformed articles, while internal nucleation and poor surface crystallization in a powder compact may result in total crystallization prior to sintering, yielding at best a weak, porous, poorly sintered article and at worst a pile of crystallized powder.
It is an object of the present invention to provide a glass-ceramic composition compatible with both forming processes. It is another object of the present invention to provide a glass-ceramic having superior mechanical strength and fracture toughness.